In general, a rolling bearing is repeatedly subject to contact stress due to rolling of the rolling elements on the races. Thus, the material of the rolling bearing is required to exhibit a sufficient hardness, withstand load and show a prolonged rolling fatigue life and a good abrasion resistance against slippage. To this end, there are used SUJ2 according to JIS as bearing steel, SUS440C according to JIS or 13Cr martensite stainless steel as stainless steel and steel material obtained by hardening or carburizing or carbonitriding steel corresponding to JIS SCR420 as case hardening steel. Having been subjected to hardening and tempering, these steel materials have a hardness HRC of from 58 to 64 to attain a sufficient rolling fatigue life.
Among rolling bearings, ball bearings for use in data device such as HDD and VTR or fan motor or the like must meet extremely severe requirements for torque, acoustic properties and noise reduction and thus are finished to an extremely high precision.
As the material of these ball bearings there is often used SUJ2, which is a high carbon-chromium bearing steel, SUS440C, which is a martensite stainless steel, 0.7C-13Cr stainless steel or the like. In order to obtain required hardness or abrasion resistance, these bearing steel materials are subjected to hardening and tempering. The resulting bearing race has a hardness HRC of from 58 to 64. It is usual that the rolling element is essentially formed by the same material as that of the races or one of the inner race and the outer race except for special cases.
In recent years, however, as devices have been smaller in size and more portable, various problems have arisen. In other words, the recent studies have shown that as devices have been smaller in size, it has been more likely that the devices can be subject to drop or vibration during transportation, causing the ball bearing incorporated therein to be damaged, though slightly, and hence causing the deterioration of the performance of the devices. Explaining further, when impact load is applied to a device, a small-sized ball bearing in particular is subject to permanent deformation of the raceway surface even under a relatively small impact load, causing deterioration of acoustic properties or uneven rotary torque resulting in the deterioration of the device comprising such a ball bearing incorporated therein, partly because the ellipsoidal area developed by the contact of the raceway surface of races with the rolling surface of rolling element is small. It is thought that this problem is attributed to the fact that the retained austenite incorporated in the steel has a low yield stress as described in JP-A-7-103241 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") . In order to reduce the content of retained austenite while maintaining hardness required for ball bearing, a countermeasure has been practiced which comprises subjecting SUJ2, for example, which has been hardened, to sub-zero treatment or tempering at a temperature of as relatively high as from 220.degree. C. to 240.degree. C. so that the content of retained austenite can be minimized or completely eliminated to prevent the deterioration of acoustic properties due to impact load.
In recent years, on the other hand, as the demand for device portability has been growing, it has been more likely that devices can be subject to vibration. As a result, minute vibration or rocking causes fretting on the contact area of the rolling element with the inner or outer race in the rolling bearing, raising a problem of deterioration of acoustic properties. The foregoing countermeasure is aimed primarily at preventing the deterioration of acoustic properties due to impact load but doesn't take into account fretting caused by the vibration or operation of the device. Further, inpractice, rolling bearings for data device such as HDD and VTR or relatively small-sized rolling bearings which are required to be still have been given countermeasure merely for lubrication but not for material.
With respect to the problem of fretting, it has been studied whether or not a ball made of ceramics such as silicon nitride can be applied (bearing comprising a ceramics ball will be hereinafter referred to as "hybrid bearing"). It has been found that since a ceramics ball exhibits good sliding properties as well as a very high hardness and thus is little liable to damage, the use of a ceramics ball as a rolling element makes it possible to inhibit cohesive abrasion and hence provide a far greater fretting durability than steel ball. Further, the past studies show that a ceramics ball gives an extremely excellent acoustic durability after continuous operation at a high rotary speed. However, a ceramics ball is disadvantageous in that its production takes a very high cost as compared with a steel ball. Further, since a ceramics ball exhibits a remarkably great elastic modulus than a steel ball, it is liable to formation of impression on the races and deterioration of acoustic properties under impact load and thus is inferior to a steel ball in impact resistance. A ceramics ball is further disadvantageous in that it exhibits a remarkably small linear expansion coefficient as compared with a steel ball and thus is liable to release of pilot pressure due to the temperature rise accompanying the actuation of the device, resulting in the reduction of rigidity. Moreover, since a ceramics is generally insulating, it can electrostatically attract foreign matters, occasionally causing dust noise that raises percent defective. Further, since a ceramics has an extremely small specific gravity and the resulting ceramics ball is light itself, the resulting electrostatic action can cause troubles in conveyance at the assembly step.